a) Field of the Invention
The invention is directed to a high-aperture optical imaging system, particularly for microscopes, and relates primarily to a high-aperture immersion objective with apochromatic correction within a broad wavelength range. The imaging system has a large image field.
b) Description of the Related Art
Objectives with a lower magnification have a larger visual field and a numerical aperture that is generally lower. One of the main reasons for this is the dominance of visual observation in the microscope. However, since the resolving capacity of the human eye is limited, it does not make sense to furnish lower- and medium-magnification objectives with a high aperture. The gain in resolution achieved through a high aperture cannot be perceived by the human eye. Further, it is simpler in technical respects for a high-magnification objective to be outfitted with a high aperture than for a low-magnification objective because objectives of high magnification image only relatively small object fields.
Accordingly, objectives with a high numerical aperture traditionally also have high magnifications. An immersion objective with a magnification of 100× and a numerical aperture up to 1.65 is described in U.S. Pat. No. 5,659,425. Immersion objectives with a high numerical aperture and low magnification are not realized.
In recent times, visual observation has diminished in importance in many fields and applications and other channels of image recording have become increasingly important. These channels often possess the possibility of post-magnification which allows full exploitation of the resolving capacity of the objective. Therefore, in applications of this kind it is reasonable to work with objectives having both a lower magnification and a high aperture because a low magnification is equivalent to a large object field and, consequently, the frequently cumbersome changing of objectives can be dispensed with.
U.S. Pat. No. 5,982,559 describes an immersion objective for microscopes which comprises eight lens groups, has a numerical aperture of 1.3, a magnification of 40× and a flat image field and in which optical errors are extensively corrected. However, this objective has no apochromatic correction, particularly up to the infrared spectral region. Yet this characteristic is increasingly desirable in connection with two-photon applications.